Quantum dots that are nanoparticles of 10 nm or less in particle size have excellent performance of confining carriers (electrons, holes), and can thus easily produce excitons by recombination of electrons and holes. For this reason, luminescence from free excitons can be expected, and it is possible to realize luminescence which has a high luminescent efficiency and a sharp emission spectrum. In addition, the quantum dots are able to be controlled in a wide range of wavelengths by using the quantum size effect, and thus attracting attention for applications to light-emitting devices such as EL elements, light emitting diodes (LED), and semiconductor lasers.
It is considered important for this type of light-emitting device to confine and recombine carriers in the quantum dots (nanoparticles) with high efficiency, thereby increasing the luminescent efficiency. Further, a self-assembly (self-organization) method of preparing quantum dots by a dry process is known as a method for preparing the quantum dots.
The self-assembly method is a method of causing gas-phase epitaxial growth of a semiconductor layer under such a specific condition that provides lattice mismatch, and causing self-formation of three-dimensional quantum dots with the use of the strain thereof. For example, when strain is produced from a difference in lattice constant between an n-type semiconductor substrate and a p-type semiconductor layer and epitaxial growth cannot be caused, a quantum dot is formed at the site with the strain produced.
However, in the self-assembly method, quantum dots are discretely distributed on the n-type semiconductor substrate, and gaps are thus produced between the adjacent quantum dots. For this reason, there is a possibility that holes transported from the p-type semiconductor layer will be transported toward the n-type semiconductor substrate without being injected into the quantum dots, or electrons transported from the n-type semiconductor substrate will be transported to the p-type semiconductor substrate without being injected into the quantum dots, and there is a possibility of causing a decrease in luminescent efficiency.
Moreover, in the self-assembly method mentioned above, there is a possibility that carriers that are not injected into the quantum dots will recombine to produce luminescence outside the quantum dots. Then, when carriers recombine to produce luminescence outside the quantum dots in such a manner, there is a possibility of producing more than one intensity peak and causing a decrease in purity of luminescent color. In addition, even when carriers that are not injected into the quantum dots recombine outside the quantum dots, the recombination does not produce luminescence and may result in non-luminescent recombination centers, and in such cases, electrical energy is released as thermal energy without being converted to light energy, and there is thus a possibility of causing a further decrease in luminescent efficiency.
Therefore, Patent Document 1 proposes a semiconductor device including a substrate with a main surface composed of a first semiconductor, a plurality of quantum dots discretely distributed on the main surface, a coating layer composed of a second semiconductor formed on the surface with the quantum dots distributed, and a barrier layer formed from a third semiconductor or an insulating material that is disposed on at least a part of the region without the quantum dots disposed in the plane with the quantum dots distributed and that has a larger bandgap than the bandgaps of the first and second semiconductors.
That is, in Patent Document 1, as illustrated in FIG. 18, n-type GaAs (first semiconductor) is used to form a substrate 101, and p-type GaAs (second semiconductor) is used to form a coating layer 102. In addition, quantum dots 103 composed of InGaAs are discretely distributed on the substrate 101 with the use of a self-assembly method, AlAs (third semiconductor) that has higher bandgap energy than GaAs is further epitaxially grown on the substrate 101 with the use of a molecular beam epitaxy method, and thereafter the AlAs is oxidized to form an insulating barrier layer 104.
In such a manner, in Patent Document 1, the gaps between the quantum dots 103 are filled with the insulating barrier layer 104 to thereby make carriers easy to inject into the quantum dots 103, and promote the recombination of electrons and holes in the quantum dots 103, thereby making an improvement in luminescent efficiency.
On the other hand, Patent Document 2 and Patent Document 3 are known as techniques of preparing colloidal quantum dots by a wet process.
Patent Document 2 proposes a light-emitting device including a light-emitting layer composed of quantum dots and emitting light by recombination of electrons and holes, an n-type inorganic semiconductor layer that transports the electrons to the light-emitting layer, a p-type inorganic semiconductor layer that transports the holes to the light-emitting layer, a first electrode for injecting the electrons into the n-type inorganic semiconductor layer, and a second electrode for injecting the holes into the p-type inorganic semiconductor layer.
In Patent Document 2, as illustrated in FIG. 19, an n-type semiconductor layer 111 and a p-type semiconductor layer 112 are formed from inorganic materials that have a band structure with favorable carrier transport properties, and a quantum dot layer 113 is interposed between the n-type semiconductor layer 111 and the p-type semiconductor layer 112.
Then, electrons transported from the n-type semiconductor layer 111 and holes transported from the p-type semiconductor layer 112 are, due to the tunnel effect, injected into the quantum dot layer 113 through potential barriers between the quantum dot layer 113 and the carrier transport layers (the n-type semiconductor layer 111 and the p-type semiconductor layer 112), thereby improving the efficiency of injecting carriers into the quantum dot layer 113.
In addition, Patent Document 3 proposes a nanoparticle luminescent material composed of a core part composed of a nanoparticle and a shell part composed of at least two types of ligands localized on the surface of the core part, where at least one of the ligands is a hole transporting ligand, and at least one thereof is an electron transporting ligand.
In Patent Document 3, with the use of a surfactant with a hole transporting ligand and an electron transporting ligand, the energy levels of each ligand are designed for such a combination that produces a carrier block effect so that carriers are confined in the nanoparticle.
FIG. 20 is a band structure diagram illustrating the energy band in Patent Document 3, where a nanoparticle has a core-shell structure.
That is, the nanoparticle 121 is composed of a core part 122 and a shell part 123 coating the core part 122, and the shell part 123 is coated with a surfactant 124. This surfactant 124 has a hole transporting ligand 124a and an electron transporting ligand 124b, and the hole transporting ligand 124a is localized closer to a hole transport layer 125, and the electron transporting ligand 124b is localized closer to an electron transport layer 126.
In Patent Document 3, the LUMO level 127 of the hole transporting ligand 124a is made higher than the LUMO level 128 of the electron transporting ligand 124b, thereby injecting electrons from the electron transport layer 126 into the core part 122, whereas the LUMO level 127 of the hole transporting ligand 124a is made higher than the lowest electron level 129 in the conduction band (for electron transfer) of the core part 122, thereby causing the hole transporting ligand 124a to serve as a barrier to electrons, and thus, electrons are confined within the core part 122.
Furthermore, in Patent Document 3, the HOMO level 130 of the electron transporting ligand 124b is made lower than the HOMO level 131 of the hole transporting ligand 124a, thereby injecting holes from the hole transport layer 125 into the core part 122, whereas the HOMO level 130 of the electron transporting ligand 124b is made lower than the highest electron level 132 in the conduction band (for hole transfer) of the core part 122, thereby causing the electron transporting ligand 124b to serve as a barrier to holes, and thus, holes are confined within the core part 122.
Here, the LUMO level refers to an energy level corresponding to the lowest unoccupied molecular orbital of molecular orbitals that are not occupied by electrons in the case where molecules irradiated with light bring energy into an excited state and bring the molecular orbitals into an empty state occupied by no electron.
In addition, the HOMO level refers to an energy level corresponding to the highest occupied molecular orbital of molecular orbitals in a ground state in the case where electrons occupy the molecular orbitals in order from the molecular orbitals with the lowest energy in the ground state before molecules are irradiated with light.
In such a manner, in Patent Document 3, due to the electron block effect of the hole transporting ligand 124a and the hole bock effect of the electron transporting ligand 124b, carriers (electrons and holes) are confined within the core part 122 of the nanoparticle 121.
Then, electrons and holes are confined within the core part 122 in such a manner to thereby recombine electrons and holes in the core part 122 and cause excitons to produce luminescence.                Patent Document 1: Japanese Patent Application Laid-Open No. 2002-184970 (claim 1, FIG. 1)        Patent Document 2: Japanese Patent Application Laid-Open No. 2006-185985 (claim 1, FIG. 1)        Patent Document 3: Japanese Patent Application Laid-Open No. 2008-214363 (claims 1 to 5)        